The regulation of the biosynthesis of choline phosphoglycerides during lung development in the rat is investigated. Emphasis is placed on the reactions involved in the cytidine nucleotide dependent incorporation of choline. The enzyme CTP: choline phosphate cytidyltransferase in fetal lung occurs primarily in a low molecular weight form and in a high molecular weight form in adult lung. Specific phospholipids are believed to be required for the conversion from inactive low molecular weight form to active high molecular weight form. The mechanisms associated with this phenomena will be studied in detail using gel filtration and sucrose density centrifugation as tools to analyze both the conditions for maximal conversion and activation and the relationship between phospholipid binding and activation. The incorporation of choline into phosphatidylcholine depends upon the availability of 1,2-diacylglycerol. The concentration of 1,2-diacylglycerol in lung during development will be determined. The dynamics of 1,2-diacylglycerol will also be investigated by comparing the relative activities of formation via phosphatidic phosphohydrolase and the utilization by CDP-choline: diglyceride transferase, CDP-ethanolamine:diglyceride transferase and acylCoA:diglyceride transferase. In addition, the turnover of diglyceride will be measured with lung slices from both adult and fetal rats. The resulting data will be used to assess the flow of the CDP-choline pathway under conditions of low synthetic rate (fetal) versus high synthetic rate (adult) and to determine the parameters associated with the increase in diglyceride utilization for phosphatidylcholine synthesis.